A man purported to be the reclusive leader of the militant Islamic State has made what would be his first public appearance at a mosque in the centre of Iraq's second city, Mosul, according to a video recording posted on the Internet on Saturday.

There had previously been reports on social media that Abu Bakr al-Baghdadi would make his first public appearance since his Islamic State in Iraq and the Levant (ISIL) changed its name to the Islamic State and declared him caliph - a title held by successors of the Prophet Mohammad.

The Iraqi government denied that the 21-minute video, which carried Friday's date, was credible.

"We have analysed the footage ... and found it is a farce," Interior Ministry spokesman Saad Maan told Reuters.

Maan said government forces had recently wounded Baghdadi in an air strike and that he had been transferred by Islamic State militants to Syria for medical treatment. But he declined to give further details and there was no way to confirm the claim independently.

If Baghdadi did preach in Mosul on Friday, it would be the first public appearance by the head of the militant group that led a military offensive last month that saw swathes of northern Iraq fall to ISIL and other Sunni armed factions. The campaign started on June 10 in Mosul, the north's biggest city, which was quickly taken by ISIL.

The recording showed a bearded man in a black robe and black turban slowly ascending the pulpit below the black flag of the Islamic State, before delivering a sermon and leading prayers. The recording called him "Caliph Ibrahim, emir of the faithful in the Islamic State, may God protect him".

It was not immediately possible to confirm the identity of the man in the recording as only one grainy picture of the reclusive Baghdadi has previously been published.

It was also not possible to immediately confirm the authenticity of the recording or the date when it was made.

"God has granted your brothers, the Mujahideen, victory and a conquest after years of patience and holy struggle, and enabled them to achieve their objective," the preacher tells the congregation in the recording.

"And they have rushed to declare an Islamic caliphate and to appoint an imam (leader), which is a duty for Muslims, a duty that had been lost for centuries that had been absent from reality, making many Muslims ignorant of it," he added.

"I have been burdened by this great affair, I have been burdened by this trust, a heavy trust," the man said. "I was appointed to be in charge of you, though I am not the best or better than you, so if you see me in the right, then help me, and if you see me in the wrong, advise me and put me right."

In an audio tape released last week, a voice purporting to be Baghdadi called on Muslims worldwide to take up arms and flock to the 'caliphate' it has declared on captured Syrian and Iraqi soil.

The video was shared widely on social media, and some residents of Mosul told Reuters on Saturday that they had witnessed a man introduced to them as Baghdadi preaching in a mosque in the centre of the city on Friday.

The three witnesses said the man had entered the mosque flanked by gunmen wearing uniforms worn by Islamic State militants.

"We held our breath out of fear and surprise," said one worshipper who spoke on condition of anonymity.

Another eyewitness said: "A man from the group started to speak to us in a loud tone in eloquent Arabic saying that Ameer al-Mumineen (Prince of Believers) Abu Bakr al-Baghdadi is here to give Friday's speech, and he asked everybody not to use mobiles to take photos or film, for security reasons."

A third worshipper added: "The speech lasted for around 20 minutes, then the man wearing black who was introduced to us as al-Baghdadi took the lead in the Friday prayer and then, after finishing, he left with dozens of his followers in a long motorcade."-Reuters